Measurement optics in a polarization-based or multiplexed heterodyne interferometer such as used for precision measurements in semiconductor device manufacturing equipment generally use a light beam including orthogonal polarization components that have different frequencies. In heterodyne interferometry, a dual frequency/dual polarization source of light is used. The frequency difference between the two orthogonally polarized beam components is important because it can be the limit to how fast something can move and the distance still be measured accurately by this type of measurement system. Zeeman split HeNe lasers can provide orthogonally polarized light components, but the difference frequency is limited to a maximum of about 8 MHz. A two-mode frequency stabilized HeNe laser can also provide two orthogonally polarized beams with frequency separation, but this frequency difference is in the 500-1500 MHz range and cannot be easily utilized by the processing electronics. The desired frequency range that will fulfill the lithography industries need for speed, but is compatible with current electronic technology is about 7-30 MHz.
Several methods of producing a desired frequency split in a heterodyne interferometer have been used in the past. Most of these prior solutions involve conditioning the light to get the desired frequency after the stabilized laser source. One prior solution is to use two high frequency acousto-optic modulators (AOMs) to generate the desired difference frequency. The laser source beam is split into two beams of orthogonal polarization. Each linearly polarized beam is sent through an AOM. The first order diffracted beams from each AOM are redirected using mirrors and recombined using a second beam splitter to become collinear and co-bore again. While the absolute frequency of the AOMs in this prior solution is typically too high to be ideal (e.g., 80 MHz) the difference in frequency between the two different AOMs can be adjusted (e.g., one at 80 MHz and the other at 90 MHz) so that when the two orthogonal, linearly polarized beam components are recombined, they have the desired difference frequency. Unfortunately, this is a more costly solution, because two AOMs are used to achieve the desired results (along with a beam splitter, two turning mirrors and a second beam splitter which acts as a beam recombiner). Other solutions using two AOMs exist, but all have the disadvantage of multiple components (e.g., minimum of two AOM units and a beam splitter), which tends to increase the cost of the solutions.
Another prior approach is to use a single low frequency isotropic AOM with a single acoustic wave and a birefringent recombination prism. While this method reduces the number of components as compared to the previously described two-AOM solution, it has significant issues of its own. The major disadvantages include: a significant portion of the source light is discarded, (even with a single polarization output laser); the solution takes a lot of space to accomplish; and the solution does not fully accomplish a secondary benefit of AOM frequency shifters in providing isolation for the laser because it only isolates feedback on one polarization. In this prior method, only a single polarization and frequency are desired prior to the AOM device, so for a Zeeman split HeNe laser, a polarizer is typically used to filter out the other polarization/frequency component from the source laser. Thus, half the source light is eliminated before the beam enters the AOM.
In the isotropic acoustic wave interaction of this prior solution, there is no effect on the beam's polarization, so the diffracted (first order), frequency shifted beam is the same polarization as the zero order or un-diffracted beam. Exiting the AOM, the zero order and first order beams have a frequency difference of around 20 MHz in a current device on the market. The job of making the beams collinear again is accomplished by passing the beams through a birefringent recombination prism. The beam separation angle exiting this type of AOM is small, so no compensation is made for making the beams co-bore again after they are made parallel with the recombination prism. Typically, the optic axis of the recombination prism is at a forty-five degree angle to the polarization of the beams. The recombination prism splits each beam into two orthogonally polarized components. One component sees the index of refraction of ne and the other component sees the index of refraction of no. The two beams refract differently at the entrance and exit prism/air interfaces due to this index difference. The apex angle of the prism is optimized to allow one polarization component of each beam to become parallel again. The other two unwanted polarized beams exiting the recombination prism are not parallel to the desired beams and are apertured. This recombination scheme effectively throws away half the optical power in the first and zero order beams. The net result is that three-fourths of the original source optical power for a Zeeman split laser (more if the AOM device operates in the Raman Nath regime) is lost using this prior single isotropic AOM method of increasing the frequency split.
It is desirable to have a small footprint or package for a heterodyne interferometry light source, as this light source is often installed in a customer's equipment. The single low-frequency isotropic AOM solution has issues that demand more space than desired. To get adequate efficiency for a low frequency isotropic AOM, a long interaction length is necessary, so the device itself is quite long. Also, the separation angle between the diffracted orders on this device is small, so a long distance is typically used to get adequate beam separation to aperture off the unwanted beams following the recombination prism. Thus, a long footprint, additional optics to focus the light to a pinhole spatial filter, or additional optics to fold the beam path in the package may be used to address this issue.
In addition, when using a single low frequency isotropic AOM with zero and first order beams, the zero order beam does not protect the laser from feedback because the frequency in that path is still the laser frequency (not shifted up or down). Reflections from this beam upstream that make it back to the laser will cause wavelength stability problems and a possible loss of lock for the laser.
In another prior approach, two shifted frequency beams are generated in the same isotropic AOM. The frequency shifts for both beams are accomplished in a couple of different ways. The first is to use one acoustic wave in the AOM. There is a polarizing beam splitter before (or attached to the AOM) to split a single frequency polarized beam into two orthogonally polarized beams. The polarizing beam splitter also does the task of orienting the two orthogonally polarized beams at the plus and minus Bragg angle of the AOM device so that one beam is up-shifted and one beam is down-shifted by the single acoustic wave frequency. The AOM itself is isotropic and does not affect the polarization of the beams. The frequency difference between the output beams is two times the AOM frequency. In another form of the single isotropic AOM solution, a longer crystal is used, and each polarized beam traverses through two acoustic waves in series, which are generated by two transducers of the AOM. The net result on the output beams is a frequency difference of two times the difference in frequency of the two AOM transducers. Again, this is an isotropic interaction (i.e., it does not affect polarization), and a beam splitter is used before the AOM device to generate two beams of orthogonal polarization and moving in diverging directions.